Heatsink and fabrication method thereof

ABSTRACT

A method of fabricating a heatsink including a substrate of a sintered compact containing Cu and W, and a thin diamond film layer formed on the surface of the substrate with good adherence, involves immersing the substrate in acid to reduce the Cu content of a surface region thereof and to roughen exposed W at that surface region, and then forming the thin diamond film layer on that surface region by vapor synthesis. Alternatively, a thin diamond film layer is formed on a surface of a porous body substrate, and then a hole in the porous body substrate is filled with Cu.

CROSS REFERENCE TO RELATED APPLICATION

This application is a Divisional of our U.S. application Ser. No.09/232,011, filed Jan. 14, 1999, now U.S. Pat. No. 6,361,857, issuedMar. 26, 2002.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a heatsink and a fabrication methodthereof. Particularly, the present invention relates to a heatsink onwhich is mounted a semiconductor element of relatively great heatgeneration such as a laser diode, a CPU (central processing unit), a MPU(microprocessor unit), a high frequency amplifier device, and the like,having a multilayer structure of a diamond layer and a metal layer, anda method of fabricating such a heatsink.

2. Description of the Background Art

The aforementioned high power semiconductor devices generate a great,amount of heat during operation. The heat generated by thesesemiconductor elements has become greater in accordance with improvementin the output and the operating frequency. The need of compact andlight-weight electronic equipment is great in the industry while thepackaging density of the semiconductor element is continuouslyincreasing. The increase in the heat generation and packaging density ofsemiconductor elements implies more stringent requirements with respectto the heat radiation characteristic of the heatsink employed in modulesin which high power semiconductor elements are mounted.

Regarding such heatsinks that require great heat dissipation, asemiconductor element is mounted on a heatsink formed of a material ofhigh thermal conductivity to prevent the semiconductor element frombecoming too hot. For heatsinks that incorporate high powersemiconductor elements such as a high power transistor or microwavemonothylic IC (MMIC) of great heat generation, beryllium oxide (BeO),for example, superior in thermal conductivity and dielectric property isconventionally used widely.

Diamond is known as the substance having the highest thermalconductivity. Research has been effected to apply diamond to theheatsink that is used for incorporating a semiconductor element.

As a heatsink employing diamond, development is in progress of aheatsink formed entirely of diamond, and a heatsink having a diamondfilm formed on a metal substrate.

Since natural diamond is precious and artificial diamond is costly, thecost of the heatsink will increase if the amount of diamond thereinbecomes greater. Therefore, a heatsink formed entirely of diamond isused with respect to a semiconductor element of high heat generationsuch as a high power laser only in the application where heat radiationis so insufficient that it prevents exhibition of proper performancewhen a substitute is used or in the application such as during the stageof research where the cost is not yet estimated. A heatsink having adiamond film formed on a metal substrate is used in products that musthave the cost reduced.

By using a heatsink formed partially of metal, the cost can be decreasedalthough the thermal conductivity is degraded in comparison to aheatsink formed only of diamond. Therefore, the cost and performance ofthe heatsink is substantially proportional. It can be said that aheatsink of higher thermal conductivity becomes more expensive.Therefore, there is a demand for an economical heatsink of high thermalconductivity.

In response to such demands, a heatsink of a multilayered structure witha thin diamond film formed on a metal of favorable thermal conductivityis disclosed in, for example, Japanese Patent Laying-Open No. 5-326767.

Conventionally, BeO superior in thermal conductivity has been widelyused for the heatsink. However, the level of the heat radiationcharacteristic that is currently required has become so high that evenBeO is even not sufficient. An approach has been made to reduce thethickness of the BeO substrate to reduce thermal resistance. However, itis difficult to process BeO per se. Furthermore, BeO is toxic. It can besaid that reduction in the thickness has come to its limit.

As to the heatsink disclosed in the above publication, copper andcopper-tungsten alloy which are metals of favorable thermal conductivityare mentioned as the substance of the substrate. These materials aresuitable for the heatsink since the thermal conductivity thereof is highcomparable to other metal materials the cost is relatively low.

However, there was problem that it is difficult to grow a thin diamondfilm on a substrate that includes copper in favorable adherence sincethe copper in the substrate does not produce carbide, does not absorbcarbon, and is not occluded with carbon, as described in New Diamond,Vol. 10, No. 3 (34), pp. 26 and 27.

Copper has a high thermal expansion coefficient whereas diamond has alow thermal expansion coefficient. Therefore, there is a problem thatthe thin diamond film will peel off the substrate as the temperature ofthe heatsink becomes higher due to the difference in thermal expansioncoefficient between copper and diamond.

If the difference in thermal expansion between the substrate and thediamond is small, warping in the diamond heatsink will not occur. Onlystress will be generated within the thin diamond film even when theheatsink attains high temperature. However, the thermal expansion ofcopper or a sintered compact including copper is greater than that ofdiamond, resulting in the problem of warping in the heatsink.

SUMMARY OF THE INVENTION

In view of the foregoing, an object of the present invention is toprovide a heatsink that can have a thin diamond film formed in goodadherence on a substrate of favorable thermal conductivity.

Another object of the present invention is to provide a heatsink thatcan have occurrence of warping suppressed.

According to an aspect of the present invention, a heatsink includes asubstrate of a sintered compact including Cu and W, and a thin diamondfilm layer formed on the surface of the substrate. The Cu content in thesubstrate is at least 5% by weight. In an X-ray diffraction chartobtained by irradiating a thin diamond film layer with an X-ray, thediffraction peak intensity of the (110) plane of W is at least 100 timesthe diffraction peak intensity of the (200) plane of Cu.

In such a heatsink, the amount of W at the surface of the substrate isrelatively great whereas the amount of Cu at the surface of thesubstrate becomes relatively smaller. Therefore, the adherence betweenthe substrate and the thin diamond film layer formed on the surface ofthe substrate is improved. As a result, the heat locally generated fromthe semiconductor element mounted on the thin diamond film layer can bepromptly dissipated at the in-plane of the thin diamond film layer byvirtue of the effect of the thin diamond film layer as a heat spreader(effect of heat dissipation) to be conveyed to the substrate. Thethermal conductivity of the substrate is increased since the Cu contentin the substrate is at least 5% by weight.

In an X-ray diffraction chart obtained by irradiating the thin diamondfilm layer with an X-ray, it is preferable that a peak of WC (tungstencarbide) appears. In this case, the adherence between the thin diamondfilm layer and the substrate is improved.

According to another aspect of the present invention, a heatsinkincludes a substrate of a sintered compact including Cu and W, and athin diamond film layer formed on the surface of the substrate. The Cucontent in the substrate is at least 5% by weight. In an X-raydiffraction chart obtained by irradiating the thin diamond film layerwith an X-ray, the diffraction peak intensity of the (211) plane of W isat least 30 times the diffraction peak intensity of the (200) plane ofthe Cu.

In such a heatsink, the amount of W at the surface of the substratebecomes relatively greater whereas the amount of Cu at the surface ofthe substrate becomes relatively smaller. Therefore, the adherencebetween the substrate and the thin diamond film layer formed on thesurface of the substrate is improved. As a result, the heat locallygenerated from the semiconductor element mounted on the thin diamondfilm layer is rapidly dissipated at the in-plane of the thin diamondfilm layer to be subsequently conveyed to the substrate by virtue of theeffect of the thin diamond film layer as a heat spreader (effect of heatdissipation). Also, the thermal conductivity of the substrate becomeshigher since the Cu content in the substrate is at least 5% by weight.

In an X-ray diffraction chart obtained by irradiating the thin diamondfilm layer with an X-ray, it is preferable that a peak of WC (tungstencarbide) appears. In this case, the adherence between the thin diamondfilm layer and the substrate is improved.

According to a further aspect of the present invention, a heatsinkincludes a substrate including Cu and a metal of a low thermal expansioncoefficient, and a thin diamond film layer formed on the surface of thesubstrate. The Cu content in the substrate is at least 5% by weight. TheCu content in the substrate becomes lower as a function of approachingthe surface of the substrate.

In the heatsink of the above structure, the Cu content at the surface ofthe substrate is lowest. Therefore, the adherence between the substrateand the thin diamond film layer on the substrate is improved since theamount of Cu that does not easily attach to carbon is small at thesurface of the substrate. As a result, the heat locally generated fromthe semiconductor element is rapidly dissipated at the in-plane of thethin diamond film layer to be subsequently conveyed to the substrate byvirtue of the thin diamond film layer serving as a heatsink spreader(effect of heat dissipation). Also, the thermal conductivity of thesubstrate is increased since the Cu content in the substrate is at least5% by weight.

The Cu content at a region of the substrate that is not more than 10 μmin depth from the surface of the substrate is preferably not more than50% of the entire Cu content of the substrate. By adjusting the contentamount of Cu at the surface of the substrate, adherence between the thindiamond film layer and the substrate is improved. If the Cu content at aregion that is not more than 10 μm in depth from the surface of thesubstrate exceeds 50% of the entire Cu content of the substrate, therate of presence of Cu becomes so high that the thin diamond film layeris easily peeled off the substrate. By setting the Cu content at thesurface of the substrate to be less than 50% of the entire Cu content ofthe substrate, warping in the substrate can be suppressed due to anappropriate amount of Cu remaining in the substrate.

The substrate is preferably a Cu—W sintered compact or a Cu—W—Mosintered compact The Cu—W sintered compact or the Cu—W—Mo sinteredcompact must have a thermal conductivity of at least 100 W/m·K in orderto exhibit the advantage of the present invention.

W particles are exposed at the surface of the substrate. The surfaceroughness R_(Z) of the W particle is preferably at least 0.05 μm. Thediamond nucleus is easily generated from the convex portion of the Wparticle. By setting the surface roughness R_(Z) of the W particle asabove, the nucleus generation density of diamond is improved. Therefore,the number of contact points between the substrate and the thin diamondfilm layer is increased. Thus, adherence between the thin diamond filmlayer and the substrate can further be improved.

If the surface roughness R_(Z) of the W particle is less than 0.05 μm,the nucleus generation density is reduced since the convex portion inthe W particle is reduced. This means that the adherence between thethin diamond film layer and the substrate is degraded so that the thindiamond film layer easily peels off the substrate.

Preferably, an intermediate layer in which the Cu content isapproximately 0% by weight is formed between the surface of thesubstrate and the thin diamond film layer. By provision of anintermediate layer that does not include Cu between the substrate andthe thin diamond film layer, the thin diamond film layer will not comeinto contact with the Cu in the substrate. Therefore, adherence betweenthe thin diamond film layer and the substrate is further improved.

The thickness of the substrate is preferably at least 200 μm and notmore than 10000 μm. In order to maintain the strength as a substrate,the thickness of the substrate is preferably at least 200 μm. In orderto avoid the thermal resistance of the heatsink from becoming too great,the substrate thickness is preferably not more than 10000 μm.

The thickness of the thin diamond film layer is preferably at least 10μm. In this case, the thin diamond film layer functions to diffusein-plane the heat generated by the semiconductor element to prevent theheat from being partially confined. The thickness of the thin diamondfilm layer must be at least 10 μm for this function.

The thermal conductivity of a thin diamond film layer is generally inthe range of 500 W/m·K-2000 W/m·K depending upon the quality of thediamond. In the present invention, the thermal conductivity of the thindiamond film layer must be at least 700 W/m·K in order to exhibit theeffect of the present invention.

Preferably, the substrate has a thermal conductivity of at least 100W/m·K, and a thickness of at least 200 μm and not more than 700 μm,whereas the thin diamond film layer has a thickness of at least 10 μmand not more than 200 μm. Here, the thin diamond film layer expandsanalogous to the substrate since it is very thin. Therefore, theexpansion of the thin diamond film layer and that of the semiconductorelement provided thereon is substantially equal. Accordingly, crackingin the semiconductor element can be suppressed.

Furthermore, the thermal conductivity of the thin diamond film layer ispreferably at least 1000 W/m·K.

A method of fabricating a heatsink according to the present inventionincludes the steps of reducing the Cu content at the surface layerregion of the substrate by immersing the surface of the substrateincluding Cu and a metal of a low thermal expansion coefficient in acid,and roughening the surface of the exposed metal of a low thermalexpansion coefficient, and forming by vapor synthesis a thin diamondfilm layer on the surface of the substrate subjected to acid treatment.

According to the heatsink fabrication method including the above steps,the Cu content at the surface of the substrate is reduced by subjectingthe surface of the substrate to acid treatment. Then, a thin diamondfilm layer is formed on the surface. Therefore, a thin diamond filmlayer can be formed on the substrate in good adherence.

The step of reducing the Cu content at the surface layer region of thesubstrate includes roughening the exposed surface of the metal of a lowthermal expansion coefficient. Therefore, adherence between the thindiamond film layer and the substrate is further improved since the thindiamond film is formed on the roughened metal of low thermal expansioncoefficient.

The acid is preferably a solution selected from the group consisting ofhydrochloric acid, nitric acid, sulfuric acid, hydrofluoric acid (HF),hydrogen peroxide (H₂O₂) and chromic acid, or a mixed solution thereof.By using these acids, the surface of the substrate can be appropriatelyroughened to facilitate formation of the thin diamond film layer.

The step of reducing the Cu content at the surface layer region of thesubstrate preferably includes a first acid treatment step of immersingthe surface of the substrate in a certain acid, and a second acidtreatment step of immersing the substrate subjected to the first acidtreatment in an acid different from the certain acid.

In forming a diamond film by vapor synthesis, a diamond nucleus isgenerated at a deep location from the surface of the substrate. In otherwords, the root of the thin diamond film is present at a deep region inthe substrate, so that the anchor effect can be expected.

The substrate is preferably at least one type of a sintered compactselected from the group consisting of a Cu—W sintered compact, a Cu—Mosintered compact and a Cu—W—Mo sintered compact.

W particles are exposed at the surface of the substrate immersed inacid. The surface roughness R_(Z) of the W particle is preferably atleast 0.05 μm. By such a surface roughness, small particles of diamondpermeate into the surface of the W particles. A thin diamond film layeris grown with the diamond particle as the nucleus. Thus, adherencebetween the thin diamond film layer and the substrate is improved.

The acid treatment that reduces the Cu content is preferably carried outuntil the peak of Cu is not detected in an X-ray diffraction chartobtained by irradiating the surface of the substrate with an X-ray.

In this case, the Cu content at the surface of the substrate issufficiently reduced. Therefore, adherence between the substrate and thethin diamond film layer formed thereon is improved.

The acid treatment of reducing the Cu content is preferably carried outuntil the porosity of the region of the substrate that is within 30 μmin depth from the surface of the substrate is at least 5% by volume andnot more than 70% by volume, and the Cu content at the region within 30μm in depth from the surface of the substrate is not more than 50% ofthe entire Cu content of the substrate.

By setting the porosity to the above described range, the grains ofdiamond can easily permeate into the hole to facilitate nucleusgeneration.

If the porosity is less than 5% by volume, nucleus generation cannoteasily occur. If the porosity is greater than 70% by volume, the holesbecome so great that the thermal conductance is reduced. As a result,the property as a heatsink will be degraded.

Further preferably, the porosity at the region within 30 μm in depthfrom the surface of the substrate is at least 10% by volume and not morethan 50% by volume.

By setting the Cu content at the region of the substrate within 30 μm indepth from the surface of the substrate to be less than 50% of theentire Cu content of the substrate, warping of this heatsink will noteasily occur.

Preferably, a step of carrying out a process of scratching the surfaceof the substrate is included prior to formation of the thin diamond filmlayer. In this case, diamond is easily nucleated from the scratch at thesurface of the substrate. Many diamond nuclei can be generated at thesurface of the substrate to promote the speed in the growth of the thindiamond film layer. Also, the thickness of the thin diamond film layercan be made uniform.

The scratching process preferably includes the step of scratching thesurface of the substrate using diamond. In this case, diamond isattached to the surface of the substrate in the scratching step of thesurface of the substrate to become the nucleus in forming the thindiamond film layer. Therefore, growth of the thin diamond film layer canfurther be promoted.

Vapor synthesis is used in forming a thin diamond film layer on thesubstrate. This vapor synthesis includes hot filament CVD (chemicalvapor deposition), plasma CVD, flame method, and the like.

According to still another aspect of the present invention, a heatsinkincludes a substrate, and a thin diamond film layer formed on thesubstrate. The substrate includes a porous body of a low thermalexpansion coefficient, and Cu filled in the hole of the porous body. Atthe surface layer of the substrate, a thin diamond film layer is presentin the hole. The diamond film layer is permeating into the hole at asurface layer of the porous body.

In the heatsink of the above structure, the difference in thermalexpansion coefficient between the substrate and the thin diamond filmlayer formed on the substrate is small since the porous body forming thesubstrate has a low thermal expansion coefficient. Adherence between thesubstrate and the thin diamond film layer is improved since the thindiamond film layer is present at the surface layer of the porous body.As a result, the thin diamond film layer can be prevented from peelingoff the substrate even when the heatsink attains high temperature.

Preferably, the substrate has a thermal conductivity of at least 100W/m·K and a thickness of at least 200 μm and not more than 700 μmwhereas the thin diamond film layer has a thickness of at least 10 μmand not more than 200 μm. The thin diamond film layer expandssubstantially analogous to the substrate due to its small thickness.Therefore, the expansion of the thin diamond film layer is substantiallyequal to that of the semiconductor element provided thereon. As aresult, cracking in the semiconductor element can be suppressed.

Furthermore, the thermal conductivity of the thin diamond film layer ispreferably at least 1000 W/m·K.

The porous body is preferably at least one type of a sintered compactselected from the group consisting of a W sintered compact, a Mosintered compact, and a W—Mo sintered compact.

Furthermore, the porosity of the porous body is preferably at least 15%by volume and not more than 60% by volume. If the porosity of the porousbody is less than 15% by volume, the heat conductivity is reduced whenthe hole is filled with Cu. If the porosity exceeds 60% by volume, thethickness of the thin diamond film layer will become nonuniform.

A heatsink fabricating method according to the present inventionincludes the steps of forming a thin diamond film layer on the surfaceof a porous body having a low thermal expansion coefficient, and flingthe hole of the porous body with Cu after formation of the thin diamondfilm layer.

According to the heatsink fabrication method including the above steps,a diamond nucleus is generated from the surface of the porous body informing a thin diamond film layer. Therefore, the nucleus of the thindiamond film layer is generated from a deep portion remote from thesurface of the porous body. In other words, the base of the thin diamondfilm layer is present at a deep region remote from the surface of thesubstrate. Therefore, the anchor effect can be expected.

Since a substrate of a porous body with a low thermal expansioncoefficient is used, the amount of thermal expansion is small when athin diamond film layer is grown. Since the amount of thermal expansionis small, warping that occurs by the difference in thermal expansionbetween the thin diamond film layer and the porous body can besuppressed during the stage of cooling the porous body after filmgrowth.

Since Cu fills the hole of the porous body, the hole of the porous bodyis exactly filled with Cu that has favorable thermal conductivity. As aresult, the thermal conductance of the substrate is improved.Accordingly, the thermal conductance of the entire heatsink is improved.

Since the porous body is absent of Cu during the stage of forming a thindiamond film layer on the porous body, the thin diamond film layer canbe formed in favorable adherence on the porous body.

The step of forming a thin diamond film layer preferably includes thestep of forming a thin diamond film layer on the surface of the porousbody by vapor synthesis. Here, the vapor synthesis method includes thehot filament CVD (chemical vapor deposition), plasma CVD, flame method,and the like.

The porous body is preferably at least one type of a sintered compactselected from the group consisting of a W sintered compact, a Mosintered compact, and W—Mo sintered compact.

The porosity of the porous body is preferably at least 15% by volume andnot more than 60% by volume. By setting the porosity to the above range,generation of a diamond nucleus from the surface of the porous body isfacilitated. The thermal conductivity when Cu is filled is increased toimprove the thermal conductivity of the entire sink.

If the porosity is less than 15% by volume, a diamond nucleus cannot beeasily generated from a deep region in the porous body. As a result,adherence between the porous body and the thin diamond film layer isdegraded. Furthermore, the thermal conductance of the heatsink isdegraded since the amount of Cu in the hole is reduced.

If the porosity exceeds 60% by volume, it will become difficult to forma thin diamond film layer of uniform thickness on the surface of theporous body although a diamond nucleus can be generated at a deep regionof the porous body. Furthermore, although the generation density of thediamond nucleus is reduced and the grain of the diamond forming the thindiamond film layer is increased, the surface roughness of the thindiamond film layer becomes greater. Accordingly, there is a problem thatthe thickness of the thin diamond film layer is not uniform, andpolishing the thin diamond film layer is time consuming.

The step of filling the hole of the porous body with Cu preferablyincludes the step of permeating molten Cu into the hole of the porousbody.

The step of filling the hole of the porous body with Cu preferablyincludes the step of heating and melting Cu and permeating molten Cu inthe hole after the porous body is placed on solid Cu.

By placing solid Cu on a heating device such as a heater, arranging theporous body thereon with a thin diamond film layer at the top surfaceand the Cu melted, the Cu will permeate into the porous body by thecapillary action. According to this method, arrangement between Cu andthe porous body is feasible. Furthermore, since the molten Cu can besuppressed from spattering around, adherence of contaminants on thesurface of the thin diamond film layer can be prevented.

The step of filling the hole of the porous body with Cu preferablyincludes the step of heating and melting Cu to permeate the molten Cuinto the bole after solid Cu is placed on the porous body where the thindiamond film layer is formed.

Since the solid Cu placed on the porous body is melted, the Cu permeatesinto the porous body by the weight and capillary action of the Cu.Therefore, the charging rate of Cu becomes faster.

The step of filling the hole of the porous body with Cu preferablyincludes the step of storing molten Cu in a container and immersing theporous body with the formed thin diamond film layer in the molten Cu topermeate the melted Cu into the hole.

Since the porous body is immersed in the molten Cu, Cu permeates equallyfrom all the faces of the porous body except for the face where the thindiamond film layer is formed. Also, the permeation speed becomes faster.

Preferably, the step of scratching the surface of the porous body priorto formation of a thin diamond film layer is included. Since a diamondnucleus is easily generated from the scratch, many diamond nucleus aregenerated at the surface of the porous body. A thin diamond film layeris grown faster and with uniform thickness.

The scratching process preferably includes the step of scratching thesurface of the porous body using diamond. Diamond is attached to thesurface of the porous body during the stage of scratching the surface ofthe porous body. This diamond becomes the nucleus in forming a thindiamond film layer. Accordingly, the growth of the thin diamond filmlayer is facilitated.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a hot filament CVD apparatus fordiamond vapor synthesis employed in the present invention.

FIG. 2 is an X-ray diffraction chart obtained by irradiating a substrateprior to being immersed in acid with an X-ray.

FIG. 3 is a graph showing the concentration of Cu, W and C in thesubstrate prior to immersion in acid.

FIG. 4 is a graph showing the concentration of Cu, W and C in asubstrate after immersion in acid and before formation of a thin diamondfilm layer.

FIG. 5 is an X-ray diffraction chart obtained by irradiating a thindiamond film layer after formation with an X-ray.

FIG. 6 is a graph showing the concentration of Cu, W and C in asubstrate after formation of a thin diamond film layer and in the thindiamond film layer.

FIG. 7 is a schematic diagram of a microwave plasma CVD apparatus fordiamond vapor synthesis employed in the present invention.

FIG. 8 is an X-ray diffraction chart obtained by irradiating the thindiamond film layer obtained in Example 3 with an X-ray.

FIG. 9 is a scanning type electron microphotograph of a certain portionof a sample obtained by Example 3.

FIG. 10 is a scanning type electron microphotograph of another portionof a sample obtained by Example 3.

FIG. 11 is a scanning type electron microphotograph of a portion of asubstrate subjected to acid treatment according to Example 11.

FIG. 12 is a scanning type electron microphotograph of another portionof a substrate subjected to acid treatment according to Example 11.

FIG. 13 is an X-ray diffraction chart of a substrate subsequent to acidtreatment according to Example 15.

FIG. 14 is an X-ray diffraction chart of a substrate subsequent to acidtreatment according to Comparative Example 3.

FIG. 15 shows the step of a heatsink fabricating method according toExample 1 of the present invention.

FIG. 16 is a schematic diagram of a thin diamond film layer producedaccording to Example 1 of the present invention.

FIG. 17 is a schematic diagram of a thin diamond film layer formed on anintermediate layer, produced according to Example 2 of the presentinvention.

FIGS. 18, 19 and 20 are schematic diagrams showing the first, second,and third steps, respectively, of a heatsink fabricating method of thepresent invention.

FIG. 21 is a sectional view schematically showing a semiconductor moduleaccording to Example 19 of the present invention.

FIG. 22 is a sectional view schematically showing a semiconductor moduleaccording to Example 20 of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS EXAMPLE 1

Referring to FIG. 1, a hot filament CVD (chemical vapor deposition)apparatus for diamond vapor synthesis employed in the present inventionincludes a reactor 21, a gas inlet 22, gas outlet 23, an AC power supply24, a tungsten filament 25, a substrate holder 27, a cooling water inlet28, and a cooling water outlet 29.

Reactor 21 includes gas inlet 22 through which material gas isintroduced and gas outlet 23 from which the material gas or gasgenerated from the material gas is output.

Tungsten filament 25 is provided in reactor 21. Tungsten filament 25 isconnected to AC power supply 24. Tungsten filament 25 is brought to redheat by conducting current from AC power supply 24 to tungsten filament25.

A molybdenum substrate holder 27 is provided beneath tungsten filament25 to hold a substrate. Since tungsten filament 25 attains a hightemperature by being brought to red heat, this substrate holder 27 isalso increased in temperature. Substrate holder 27 is provided withcooling water inlet 28 through which cooling water to cool substrateholder 27 is input and cooling water outlet 29 from which the coolingwater is output.

A substrate formed of a Cu—W sintered compact having the Cu content of11% by weight and the size of 13.5 mm×13.5 mm×0.635 mm(lengthwise×breadthwise×thickness) was prepared. An X-ray of a CuKαvessel was directed to the surface of the substrate to obtain an X-raydiffraction chart. The result is shown in FIG. 2.

It is appreciated from FIG. 2 that the peak arising from W (tungsten) isgreater than the peak arising from Cu (copper). It is thereforeunderstood that the amount of W is great at the surface of thesubstrate. Also, the relationship between the depth from the surface ofthe substrate and the concentration of each component was evaluated. Theresult is shown in FIG. 3.

In FIG. 3, dotted line 201 indicates the actually measured value of theconcentration of Cu in the substrate. Dotted line 204 indicates anaverage of the actually measured values of the Cu concentrationindicated by dotted line 201 for every 20 μm in depth. Solid line 202indicates the actually measured value of the W concentration in thesubstrate. Solid line 205 shows the average of the measured values of Windicated by solid line 202 for every 20 μm in depth. Chain dotted line203 indicates the concentration of C inside and outside the substrate.The C concentration outside the substrate is the concentration of C inthe fixture jig to secure the substrate.

In FIG. 3, the depth from the interface is plotted along the abscissa,and an arbitrary amount is plotted along the ordinates. Therefore, theaccurate ratio of the concentration of Cu to W is not represented. Thesame applies for subsequent FIGS. 4 and 6.

It is appreciated from FIG. 3 that the concentration of W and Cu issubstantially constant within the substrate.

According to step A of FIG. 15, the above substrate was immersed forfive minutes in a solution of mixed acid (mixture of hydrofluoric acidand nitric acid in the weight ratio of 1:1) diluted with pure water. Theconcentration of Cu and W within the substrate was measured. The resultis shown in FIG. 4.

In FIG. 4, dotted line 211 shows the measured value of the Cuconcentration in the substrate. Dotted line 214 shows the average of themeasured values of Cu indicated by dotted line 211 for every 20 μm indepth. Solid line 212 shows the measured value of the W concentrationwithin the substrate. Solid line 215 shows the average of the measuredvalues of the W concentration within the substrate indicated by solidline 212 for every 20 μm in depth. Although chain dotted line 213 showsthe concentration of C outside and inside the substrate, the Cconcentration outside the substrate is the C concentration in thefixture jig that supports the substrate. It is appreciated from FIG. 4that the Cu at the surface of the substrate is removed by the mixed acidso that the Cu concentration (Cu content) becomes lower as a function ofapproaching the surface of the substrate.

Following the process of scratching the surface of the substrate withdiamond abrasive grains, a substrate 26 was placed on substrate holder27 in hot filament CVD apparatus 1 of FIG. 1. Current was conducted fromAC power supply 24 to tungsten filament 25 to set the temperature oftungsten filament 25 to approximately 2050° C.

Then, mixture gas of methane and hydrogen with the methane concentrationof 1 mole % was introduced through gas inlet 22 into reactor 21. Thepressure within reactor 21 was maintained at 70 Torr. According to stepB of FIG. 15, a thin diamond film layer was grown on substrate 26 overthe period of 40 hours. Thus, a thin diamond film layer 31 shown in FIG.16 was obtained. The obtained thin diamond film layer 31 was 24 μm inthickness. The warp of substrate 26 was 3 μm.

The surface of thin diamond film layer 31 was polished to have a mirrorface. Then, an X-ray generated from a CuKα vessel was directed onto thesurface of thin diamond film layer 31 to obtain an X-ray diffractionchart. The obtained X-ray diffraction chart is shown in FIG. 5.

Referring to FIG. 5, the ratio I_(W) (110)I_(Cu) (200) of the peakintensity (height) I_(W) (110) of the (110) plane of W to the peakintensity (height) I_(Cu) (200) of the (200) plane of Cu was 119. Theratio of the peak intensity (height) I_(W) (211) of the (211) plane of Wto the peak intensity (height) I_(Cu) (200) of the (200) plane of Cu was50. The relationship between the depth from the interface of substrate26 and thin diamond film layer 31 and the concentration of eachcomponent was evaluated. The result is shown in FIG. 6.

In FIG. 6, dotted line 221 shows the measured value of the Cuconcentration in the substrate. Dotted line 224 shows the average of themeasured values of the Cu concentration indicated by dotted line 221 forevery 20 μm in depth. Solid line 222 shows the measured value of the Wconcentration inside the substrate. Solid line 225 shows the average ofthe measured values of W indicated by solid line 222 for every 20 μm indepth. Chain dotted line 223 shows the concentration of C inside andoutside the substrate.

It is appreciated from FIG. 6 that the Cu concentration (content)becomes smaller as a function of coming closer to the surface of thesubstrate. The Cu content at the region located within 10 μm in depthfrom the surface of the substrate was not more than 50% of the entire Cucontent (11% by weight) of the substrate.

Thin diamond film layer 31 did not peel off substrate 26 even whensubstrate 26 was cut to the size of 2 mm×1 mm×0.635 mm(lengthwise×breadthwise×thickness). Then, a laser diode was mounted onthe heatsink produced by applying metallization. The laser diodeexhibited stable oscillation. It is therefore appreciated that thisheatsink is sufficient for practical usage.

COMPARATIVE EXAMPLE 1

A substrate was prepared formed of a Cu—W sintered compact with the Cucontent of 11% by weight in the size of 13.5 mm×13.5 mm×0.635 mm(lengthwise×breadthwise×thickness). This substrate was immersed for 30seconds in a solution of nitric acid diluted with pure water. A processof scratching the surface of the substrate with diamond abrasive grainswas carried out. Then, substrate 26 was placed on substrate holder 27 inhot filament CVD apparatus 1.

The temperature of tungsten filament 25 was set to approximately 2050°C. Mixture gas of methane and hydrogen with the methane concentration of1 mole % was introduced into reactor 21 through gas inlet 22. Thepressure in reactor 21 was 70 Torr. A thin diamond film layer was grownon the substrate over 40 hours under the above conditions. The thindiamond film layer was 23.5 μm in thickness and the warp of thesubstrate was 3.4 μm.

The thin diamond film layer had its surface polished to have a mirrorface and then irradiated with an X-ray generated from a CuKα vessel toobtain an X-ray diffraction chart. The ratio of the peak intensity(height) of the (110) plane of W to the peak intensity of the (200)plane of Cu was obtained from the X diffraction chart. The peakintensity ratio was 65. This substrate was cut out to the size of 2 mm×1mm×0.635 mm (lengthwise×breadthwise×thickness). It was found that thethin diamond film layer peeled off the substrate.

EXAMPLE 2

A substrate 26 was prepared formed of a Cu—W sintered compact with theCu content of 15% by weight and 13.5 mm×13.5 mm×0.635 mm in size(lengthwise×breadthwise×thickness) as shown in FIG. 17. The surface ofsubstrate 26 was roughened so that the surface roughness R_(Z) of thesubstrate was 1 μm. Here, R_(Z) refers to the ten point height ofirregularities defined by JIS (Japanese Industrial Standards).

On the surface subjected to the above roughening process, SiC 32 asshown in FIG. 17 was deposited to 3 μm in thickness as an intermediatelayer not including Cu. The surface of the intermediate layer wasscratched with diamond abrasive grains. Then, substrate 26 was mountedon substrate holder 27 in hot filament CVD apparatus 1 of FIG. 1. Thetemperature of tungsten filament 25 was set to approximately 2100° C.Mixture gas of methane and hydrogen with the methane concentration of 1mole % was introduced into reactor 21 through gas inlet 22. The pressurewithin reactor 21 was set to 70 Torr. A thin diamond film layer 31 asshown in FIG. 17 was grown on substrate 26 over 40 hours under the aboveconditions. The obtained thin diamond film layer 31 had a thickness of22 μm. The warp of substrate 26 was 2.5 μm. Thin diamond film layer 31had its surface polished to have a mirror face and then irradiated withan X-ray generated from a CuKα vessel to obtain an X-ray diffractionchart. From this X-ray diffraction chart, the ratio I_(W) (211)/I_(Cu)(200) of the peak intensity (height) I_(W) (211) by the (211) plane of Wto the peak intensity (height) I_(Cu) (200) of the (200) plane of Cu was47.

Then, the substrate was cut to the size of 2 mm×1 mm×0.635 mm(lengthwise×breadthwise×thickness). However, thin diamond film layer 31did not peel off substrate 26. Then, the plane of substrate 26 oppositeto the plane where thin diamond film layer 31 is formed was polished,and then subjected to metallization to produce a heatsink. A laser diodewas installed on the heatsink. The laser diode exhibited oscillation. Itis therefore appreciated that this heatsink is sufficient for practicalusage.

COMPARATIVE EXAMPLE 2

A substrate of 13.5 mm×13.5 mm×0.635 mm(lengthwise×breadthwise×thickness) in size was prepared, formed of aCu—W sintered compact with the Cu content of 15% by weight. The surfaceof this substrate was subjected to a roughening process so that thesurface roughness R_(Z) of the surface of the substrate was 5 μm.Following the roughening process, SiC of 12 μm in thickness wasdeposited as an intermediate layer not including Cu on the surface.

The surface of the intermediate layer was subjected to a process ofscratching the surface with diamond abrasive grains. Substrate 26 wasplaced on substrate holder 27 in hot filament CVD apparatus 1 shown inFIG. 1. The temperature of tungsten filament 25 was set to approximately2100° C. Mixture gas of methane and hydrogen with the methaneconcentration of 1 mole % was introduced into reactor 21 through gasinlet 22. The pressure in reactor 21 was set to 70 Torr. A thin diamondfilm layer was grown on the substrate over 40 hours under the aboveconditions. The thickness of the thin diamond film layer was 23 μm. Thewarp of the substrate was 3.5 μm.

The thin diamond film layer had its surface polished to result in amirror face, and then irradiated with an X-ray generated from a CuKαvessel to obtain an X-ray diffraction chart. According to this X-raydiffraction chart, the ratio I_(W) (211)/I_(Cu) (200) of the peakintensity (height) I_(W) (211) of the (211) plane of W to the peakintensity (height) I_(Cu) (200) of the (200) plane of Cu was 18. Thesubstrate was cut out to the size of 2 mm×1 mm×0.635 mm(lengthwise×breadthwise×thickness). The thin diamond film layer peeledoff the substrate.

EXAMPLE 3

FIG. 7 is a schematic diagram showing a microwave plasma CVD apparatusfor diamond vapor synthesis employed in the present invention. Microwaveplasma CVD apparatus 100 includes a substrate holder 101, a microwavepower source 104, a tuner 105, a wave guide 106, a reactor 107, anoutlet 108, an inlet 109, and a plunger 110.

Substrate holder 101 for supporting a substrate is provided withinreactor 107. Reactor 107 includes inlet 109 from which material gas isintroduced and outlet 108 from which the material gas or gas generatedby the reaction is output. Outlet 108 is connected to a vacuum pump.Microwave power source 104, an isolator (not shown) and a tuner 105 formthe microwave generation unit. Reactor 107 is formed of a quartz tube.

The microwave generated from the microwave generation unit is directedtowards plunger 110 via wave guide 106. Since reactor 107 is provided inthe passage of wave guide 106, plasma is generated within reactor 107 asindicated by the circled dotted line 103. Plasma is generated at thearea where reactor 107 crosses wave guide 106. Therefore, substrateholder 101 is provided in the proximity of this crossing position.

A substrate of 13.5 mm×13.5 mm×1 mm (lengthwise×breadthwise×thickness)was prepared, formed of a Cu—W sintered compact with the Cu content of11% by weight. This substrate was immersed for two minutes in a solutionof mixed acid (mixture of hydrofluoric acid and nitric acid in theweight ratio of 1:1) diluted with pure water. The surface of thesubstrate was subjected to the process of scratching the surface withdiamond abrasive grains. Then, substrate 102 was placed on substrateholder 101 in microwave plasma CVD apparatus 100.

The temperature of substrate 102 was set to 850° C. Mixture gas ofmethane and hydrogen with the methane concentration of 3 mole % wasintroduced through inlet 109. The pressure within reactor 107 wasmaintained at 140 Torr. Plasma was generated in reactor 107. A thindiamond film layer was formed on substrate 102 over 20 hours under theabove conditions. The thin diamond film layer had a thickness of 22 μm.The warp of the substrate was 4 μm.

The thin diamond film layer had its surface polished to have a mirrorface, and then irradiated with an X-ray generated from a CuKα vessel toobtain an X-ray diffraction chart. The obtained X-ray diffraction isshown in FIG. 8.

According to the chart of FIG. 8, the ratio I_(W) (110)/I_(Cu) (200) ofthe peak intensity (height) I_(W) (110) of the (110) plane of W to thepeak intensity (height) I_(Cu) (200) of the (200) plane of Cu was 140.The ratio I_(W) (211)/I_(Cu) (200) of the peak intensity (height) I_(W)(211) of the (211) plane of W to the peak intensity height) I_(Cu) (200)of the (200) plane of Cu was 50.

Even when a plurality of substrates were formed by cutting out thesubstrate at the size of 2 mm×1 mm×1 mm(lengthwise×breadthwise×thickness), the thin diamond film layer did notpeel off the substrate. One of the plurality of substrates had the thindiamond film layer peeled off intentionally to observe an area range of3×4 μm² of the surface of the substrate with a 3D-SEM (three dimensionalscanning electron microscope) of the ERA 8000 type from ELIONIX. Theobserved result is shown in FIG. 9.

W particles were exposed at the surface of the substrate from theobserved result. The surface roughness R_(Z) of the W particle wasmeasured. The surface roughness R_(Z) was 0.09 μm. Another area of thesurface of the substrate was observed with a FESEM (field emissionscanning electron microscope). The observed result is shown in FIG. 10.

One of the plurality of substrates cut up had the surface opposite tothe surface where the thin diamond film layer is formed polished andsubjected to metallization to produce a heatsink. A laser diode wasinstalled on this heatsink. The laser diode exhibited stableoscillation. It is therefore appreciated that this heatsink issufficient for practical usage.

EXAMPLE 4

A porous body of 10 mm×10 mm×0.3 mm (lengthwise×breadthwise×thickness)in size was prepared, formed of a W sintered compact with the porosityof 27.5% by volume. Cu was permeated into the hole of the porous body.Accordingly, the entire Cu content of the substrate was set to 10% byweight, and the Cu content at the region of 10 μm in depth from thesurface where a thin diamond film layer is to be formed was set to 3% byweight.

The surface of the substrate was subjected to a scratching process usingdiamond abrasive grains. Substrate 102 was mounted on substrate holder101 in microwave plasma CVD apparatus 100 shown in FIG. 7. Thetemperature of substrate 102 was set to 850° C. Mixture gas of methaneand hydrogen with the methane concentration of 3.5 mole % was introducedinto reactor 107 through inlet 109. The pressure in reactor 107 was setto 140 Torr. A thin diamond film layer was grown on substrate 102 over20 hours. The thin diamond film layer had a thickness of 25 μm. The warpof the substrate was 2.7 μm.

The surface of the thin diamond film layer was polished to have a mirrorface. The substrate was cut to the size of 2 mm×mm×0.3 mm(lengthwise×breadthwise×thickness). The thin diamond film layer did notpeel off the substrate. Then, the substrate was subjected tometallization to produce a heatsink. A laser diode was installed on thisheatsink. The laser diode exhibited stable oscillation. It is thereforeappreciated that this heatsink is sufficient for practical usage.

EXAMPLE 5

A substrate of 10 mm×10 mm×0.3 mm (lengthwise×breadthwise×thickness) wasprepared, formed of a Cu—W—Mo sintered compact with the Cu content of15% by weight. This substrate was immersed for three minutes in asolution of nitric acid diluted with pure water. The surface of thissubstrate was subjected to a scratching process with diamond abrasivegrains. Then, substrate 206 was mounted on substrate holder 27 in hotfilament CVD apparatus 1 shown in FIG. 1.

The temperature of tungsten filament 25 was set to approximately 2100°C. Mixture gas of methane and hydrogen with the methane concentration of2 mole % was introduced into reactor 21 through gas inlet 22. Thepressure in reactor 21 was set to 70 Torr. A thin diamond film layer wasgrown on the substrate over 40 hours under the above conditions. Thethin diamond film layer had a thickness of 22 μm. The warp in thesubstrate was 3 μm.

The thin diamond film layer had its surface polished to have a mirrorface, and then irradiated with an X-ray generated from a CuKα vessel toobtain an X-ray diffraction chart. According to this X-ray diffractionchart, the ratio I_(W) (110)/I_(Cu) (200) of the peak intensity (height)I_(w) (110) of the (110) plane of W to the peak intensity (height)I_(Cu) (200) of the (200) plane of Cu was 120.

This substrate was cut to the size of 2 mm×1 mm×0.3 mm(lengthwise×breadthwise×thickness). The thin diamond film layer did notpeel off the substrate. Then, the face of the substrate opposite to theface where the thin diamond film layer is formed was subjected tometallization to produce a heatsink. A laser diode was installed on thisheatsink. This laser diode exhibited oscillation. It is thereforeappreciated that this heatsink is sufficient for practical usage.

EXAMPLE 6

A substrate of 13.5 mm×13.5 mm×0.6 mm (lengthwise×breadthwise×thickness)was prepared formed of a Cu—W sintered compact with the Cu content of11% by weight. This substrate was immersed in aqua regia (a solutionhaving concentrated nitric acid and concentrated hydrochloric acid mixedat the volume ratio of 1:3) for approximately eight minutes. The surfaceof this substrate was subjected to a scratching process with diamondabrasive grains. Then, substrate 26 was mounted on substrate holder 27in hot filament CVD apparatus of FIG. 1.

The temperature of tungsten filament 25 was set to approximately 2000°C. Mixture gas of methane and hydrogen with the methane concentration of2 mole % was introduced into reactor 21 through gas inlet 22. Thepressure within reactor 21 was maintained at 60 Torr. A thin diamondfilm layer was grown on substrate 26 over 45 hours under the aboveconditions. The obtained thin diamond film layer had a thickness of 25μm. The warp of the substrate was 2 μm.

The thin diamond film layer had its surface polished to have a mirrorface, and then irradiated with an X-ray generated from a CuKα vessel toobtain an X-ray diffraction chart. According to the obtained X-raydiffraction chart, the ratio I_(W) (211)/I_(Cu) (200) of the peakintensity (height) I_(W) (211) of the (211) plane of W to the peakintensity (height) I_(Cu) (200) of the (200) plane of Cu was 45.

The substrate was cut to the size of 2 mm×1 mm×0.6 mmlengthwise×breadthwise×thickness). The thin diamond film layer did notpeel off the substrate. Then, the substrate was subjected tometallization to produce a heatsink. A laser diode was installed on theheatsink. This laser diode exhibited stable oscillation. It is thereforeappreciated that this heatsink is sufficient for practical usage.

EXAMPLE 7

A substrate of 13.5 mm×13.5 mm×0.635 mm(lengthwise×breadthwise×thickness) was prepared formed of a Cu—Wsintered compact with the Cu content of 11% by weight. Si of 5 μm inthickness was deposited as an intermediate layer not including Cu on thesurface of this substrate. This intermediate layer was subjected to ascratching process with diamond abrasive grains. Substrate 102 wasplaced on substrate holder 101 in microwave plasma CVD apparatus 100shown in FIG. 7.

The temperature of substrate 102 was set to approximately 900° C.Mixture gas of methane and hydrogen with the methane concentration of2.5 mole % was introduced into reactor 107 through gas inlet 109. Thepressure within reactor 107 was maintained at 100 Torr. A thin diamondfilm layer was grown on substrate 102 over 30 hours. The thickness ofthe thin diamond film layer was 23 μm. The warp of the substrate was 4μm.

The thin diamond film layer had its surface polished to have a mirrorface, and then irradiated with an X-ray generated from a CuKα vessel toobtain an X-ray diffraction chart. According to the X-ray diffractionchart, the ratio I_(W) (110)/I_(Cu) (200) of the peak intensity (height)I_(W) (110) of the (110) plane of W to the peak intensity (height)I_(Cu) (200) of the (200) plane of Cu was 130. A substrate of 2 mm×1mm×0.635 mm (lengthwise×breadthwise×thickness) was cut. The thin diamondfilm layer did not peel off the substrate. Then, the substrate wassubjected to metallization to produce a heatsink. A laser diode wasinstalled on the heatsink. This laser diode exhibited stableoscillation. It is therefore appreciated that this heatsink issufficient for practical usage.

EXAMPLE 8

A substrate of 13.5 mm×13.5 mm×0.6 mm (lengthwise×breadthwise×thickness)formed of a Cu—W sintered compact with the Cu content of 15% by weightwas prepared. This substrate was immersed for one minute in a solutionof mixed acid (mixture of hydrofluoric acid and nitric acid in theweight ratio of 1:1) diluted with pure water. The surface of thissubstrate was subjected to a scratching process with diamond abrasivegrains. Then, substrate 26 was placed on substrate holder 27 in hotfilament CVD apparatus 1 of FIG. 1.

The temperature of tungsten filament 25 was set to approximately 2100°C. Mixture gas of methane and hydrogen with the methane concentration of1 mole % was introduced into reactor 21 through gas inlet 22. Thepressure in reactor 21 was set to 70 Torr. A thin diamond film layer wasgrown on substrate 26 over 44 hours under the above conditions. The thindiamond film layer had a thickness of 21 μm. The warp of the substratewas 3 μm.

The thin diamond film layer had its surface polished to have a mirrorface, and then irradiated with an X-ray generated from a CuKα vessel toobtain an X-ray diffraction chart. According to the X-ray diffractionchart, the ratio I_(W) (110)/I_(Cu) (200) of the peak intensity (height)I_(W) (110) of the (110) plane of W to the peak intensity (height)I_(Cu) (200) of the (200) plane of Cu was 121. A plurality of WC peaksappeared in this X-ray diffraction chart.

The substrate was cut to the size of 2 mm×1 mm×0.6 mm(lengthwise×breadthwise×thickness). However, the thin diamond film layerdid not peel off the substrate. Then, the substrate was subjected tometallization to produce a heatsink. A laser diode was installed on theheatsink. This laser diode exhibited stable oscillation. It is thereforeappreciated that this heatsink is sufficient for practical usage.

EXAMPLE 9

In Example 3, the surface roughness R_(Z) of the W particle on thesurface of the substrate was set to 0.09 μm by immersing the substratein mixed acid. In Example 9, a method of roughening the surface of the Wparticle by a method other than that of the mixed acid process wasstudied. As a result, it was found that the surface of the W particlecould be roughened by the following five methods.

1: Shot blasting of spraying fine particles such as of metal on thesubstrate

2: Argon sputtering of converting argon gas into plasma, and applyingbias on the substrate to have argon atoms collide with the substrate

3: Alkali process of immersing the substrate in alkali

4: Fluorine plasma process of exposing the substrate to fluorine plasma

5: Electron beam irradiation of irradiating the substrate with anelectron beam

EXAMPLE 10

A substrate of 13.5 mm×13.5 mm×0.6 mm (lengthwise×breadthwise×thickness)in size formed of a Cu—W sintered compact with the Cu content of 11% byweight was prepared. This substrate was immersed for 30 minutes in asolution of nitric acid diluted with pure water. Then, the surface ofthe substrate was irradiated with an X-ray to obtain an X-raydiffraction chart. No Cu peak was observed in this X-ray diffractionchart.

The surface of the substrate was subjected to a scratching process withdiamond abrasive grains. Then, substrate 26 was placed on substrateholder 27 in hot filament CVD apparatus 1 shown in FIG. 1. Then, currentwas conducted from AC power supply 24 to tungsten filament 25, wherebythe temperature of tungsten filament 25 was set to approximately 2050°C.

Then, mixture gas of methane and hydrogen with the methane concentrationof 1 mole % was introduced into reactor 21 from gas inlet 22. A thindiamond film layer was grown on substrate 26 over 40 hours under theconditions that the pressure in reactor 21 was maintained at 70 Torr.The thin diamond film layer had a thickness of 24 μm. The warp of thesubstrate was 3 μm.

The thin diamond film layer was polished to have a mirror plane. Thenthe substrate was cut to the size of 2 mm×1 mm×0.6 mm(lengthwise×breadthwise×thickness). The thin diamond film layer did notpeel off the substrate.

Then, the substrate was subjected to metallization to produce aheatsink. A laser diode was installed on the heatsink. This laser diodeexhibited oscillation. It is therefore appreciated that this heatsink issufficient for practical usage.

EXAMPLE 11

A substrate of 10 mm×10 mm×0.6 mm (lengthwise×breadthwise×thickness)formed of a Cu—W sintered compact with the Cu content of 15% by weightwas prepared. This substrate was immersed for three minutes in asolution of mixed acid (hydrofluoric acid and nitric acid mixed at thevolume ratio of 1:1) with pure water. The surface roughness R_(Z) of thesubstrate was 4.5 μm. The surface of the substrate subjected to acidtreatment was observed with a 3D-SEM (three dimensional scanningelectron microscope) of the ERA 8000 type from ELIONIX. The observedresult is shown in FIG. 11.

According to FIG. 11, W particles are exposed at the surface of thesubstrate subjected to acid treatment. The surface roughness of theexposed W particle was 0.08 μm.

The surface of this substrate was observed by a FESEM (field emissionscanning electron microscope). The observed result is shown in FIG. 12.

The surface of the substrate subjected to acid treatment was scratchedwith diamond grains. Then, substrate 26 was placed on substrate holder27 in hot filament CVD apparatus 1 of FIG. 1.

The temperature of tungsten filament 25 was set to approximately 2100°C. Mixture gas of methane and hydrogen with the methane concentration of1 mole % was introduced into reactor 21 through gas inlet 22. Thepressure in reactor 21 was maintained at 70 Torr. A thin diamond filmlayer was grown on substrate 26 over 40 hours under the aboveconditions. The thin diamond film layer had a thickness of 22 μm. Thewarp of the substrate was 2.5 μm.

Then, the surface of the thin diamond film layer was polished to have amirror plane. The substrate was cut to the size of 2 mm×1 mm×0.6 mm(lengthwise×breadthwise×thickness). However, the thin diamond film layerdid not peel off the substrate.

Then, the surface of the substrate opposite to the surface where thethin diamond film layer is formed was polished and then subjected tometallization to produce a heatsink. A laser diode was placed on theheatsink. This laser diode exhibited stable oscillation. It is thereforeappreciated that this heatsink is sufficient for practical usage.

EXAMPLE 12

A substrate of 13.5 mm×13.5 mm×0.6 mm (lengthwise×breadthwise×thickness)formed of a Cu—W compact with the Cu content of 15% by weight wasprepared. This substrate was immersed in a solution of hydrogen peroxidefor one minute, then in nitric acid for five minutes. The surfaceroughness R_(Z) of the substrate was 4.5 μm. The surface of thesubstrate was subjected to a scratching process by diamond grains.Substrate 26 was placed on substrate holder 27 in hot filament CVDapparatus 1 shown in FIG. 1.

The temperature of tungsten filament 25 was set to approximately 2100°C. Mixture gas of methane and hydrogen with the methane concentration of1 mole % was introduced into reactor 21 through gas inlet 22. Thepressure in reactor 21 was set to 70 Torr. A thin diamond film layer wasgrown on substrate 26 over 40 hours under the above conditions. The thindiamond film layer had a thickness of 21 μm. The warp of the substratewas 2.5 μm.

The surface of the thin diamond film layer was polished to have a mirrorplane. The substrate was cut to the size of 2 mm×1 mm×0.6 mm(lengthwise×breadthwise×thickness). The thin diamond film layer did notpeel off the substrate. Then, the surface of the substrate opposite tothe surface where the thin diamond film layer is formed was polished andsubjected to metallization to produce a heatsink. A laser diode wasinstalled on this heatsink. This laser diode exhibited oscillation. Itis therefore appreciated that this heatsink is sufficient for practicalusage.

EXAMPLE 13

A substrate of 15 mm×15 mm×1 mm (lengthwise×breadthwise×thickness)formed of a Cu—Mo sintered compact with the Cu content of 10% by weightwas prepared. This substrate was immersed in sulfuric acid for thirtyminutes. Accordingly, the porosity at the region of the substrate within30 μm in depth from the surface of the substrate was 25% by volume, andthe Cu content at the region of the substrate within 30 μm in depth fromthe surface was 2% by weight. It is to be noted that the entire Cucontent of the substrate was still 10% by weight.

The surface of the substrate was subjected to a scratching process withdiamond grains. Then, substrate 102 was placed on substrate holder 101in microwave plasma CVD apparatus 100 shown in FIG. 4. The temperatureof substrate 102 was set to approximately 850° C. Mixture gas of methaneand hydrogen with the methane concentration of 3 mole % was introducedinto reactor 107 through inlet 109. The pressure in reactor 107 wasmaintained at 140 Torr.

A thin diamond film layer was grown on substrate 102 over 20 hours underthe above conditions. The thin diamond film layer had a thickness of 22μm. The warp of the substrate was 4 μm.

The surface of the thin diamond film layer was polished to have a mirrorplane. Then, the substrate was cut to the size of 2 mm×1 mm×1 mm(lengthwise×breadthwise×thickness). The thin diamond film layer did notpeel off the substrate. Then, the surface of the substrate opposite tothe surface where the thin diamond film layer is formed was polished andsubjected to metallization to produce a heatsink. A laser diode wasinstalled on this heatsink. This laser diode exhibited oscillation. Itis therefore appreciated that this heatsink is sufficient for practicalusage.

EXAMPLE 14

A substrate of 10 mm×10 mm×0.3 mm (lengthwise×breadthwise×thickness) insize formed of a Cu—W sintered compact with the Cu content of 15% byweight was prepared. This substrate was immersed in hydrochloric acidfor forty minutes. The surface roughness R_(Z) of the substrate resultedin 3.6 μm.

The surface of the substrate was subjected to a scratching process withdiamond grains. Substrate 102 was placed on substrate holder 101 in themicrowave plasma CVD apparatus shown in FIG. 7. The temperature ofsubstrate 102 was set to approximately 850° C. Mixture gas of methaneand hydrogen with the methane concentration of 3.5 mole % was introducedinto reactor 107 through inlet 109. The pressure in reactor 107 wasmaintained at 140 Torr. A thin diamond film layer was grown on substrate102 over 20 hours under the above conditions. The thickness of the thindiamond film layer was 25 μm. The warp of the substrate was 2.7 μm.

The surface of the thin diamond film layer was polished to have a mirrorplane. Then, the substrate was cut to the size of 2 mm×1 mm×0.3 mm(lengthwise×breadthwise×thickness). The thin diamond film layer did notpeel off the substrate. Then, the surface of the substrate opposite tothe surface where the thin diamond film layer is formed was polished andsubjected to metallization to form a heatsink. A laser diode wasinstalled on this heatsink. This laser diode exhibited oscillation. Itis therefore appreciated that this heatsink is sufficient for practicalusage.

EXAMPLE 15

A substrate of 10 mm×10 mm×0.6 mm (lengthwise×breadthwise×thickness) insize formed of a Cu—W sintered compact with the Cu content of 11% byweight was prepared. This substrate was immersed for one minute in asolution of mixed acid (mixture of hydrofluoric acid and nitric acid inthe weight ratio of 1:1) diluted with pure water. Then, the substratewas immersed for five minute in nitric acid. The substrate wasirradiated with an X-ray generated from a CuKα vessel to obtain an X-raydiffraction chart. This X-ray diffraction chart is shown in FIG. 13.

It is appreciated from FIG. 13 that no Cu peak appears in the X-raydiffraction chart of the surface of the substrate subjected to acidtreatment.

Then, the surface of this substrate is subjected to a scratching processusing diamond grains. Then, substrate 26 was placed on substrate holder27 in hot filament-CVD apparatus 1 shown in FIG. 1. Then, temperature oftungsten filament 25 was set to approximately 2100° C. Mixture gas ofmethane and hydrogen with the methane concentration of 1 mole % wasintroduced into reactor 21 through gas inlet 22. The pressure of reactor21 was set to 70 Torr. A thin diamond film layer was formed on substrate26 over 40 hours under the above conditions. The thickness of the thindiamond film layer was 20 μm. The warp of the substrate was 2.5 μm.

The surface of the thin diamond film layer was polished to have a mirrorplane. Then the substrate was cut to the size of 2 mm×1 mm×0.6 mm(lengthwise×breadthwise×thickness). The thin diamond film layer did notpeel off the substrate. Then, the surface of the substrate opposite tothe surface where the thin diamond film layer is formed was polished sothat the thickness of the substrate was 0.3 mm. This substrate wassubjected to metallization to produce a heatsink. A laser diode wasinstalled on the heatsink. This laser diode exhibited oscillation. It istherefore appreciated that this heatsink is sufficient for practicalusage.

COMPARATIVE EXAMPLE 3

In Comparative Example 3, the condition of the acid treatment of Example15 was altered. In contrast to Example 15 in which the substrate wasimmersed for one minute in mixed acid diluted with pure water and thenimmersed for five minutes in nitric acid, Comparative Example 3 had thesubstrate immersed for ten seconds in mixed acid diluted with purewater, but was not immersed in nitric acid thereafter. A thin diamondfilm layer was grown on the substrate subjected to acid treatment underthe conditions similar to those of Example 15. This thin diamond filmlayer was irradiated with an X-ray generated from a CuKα vessel toobtain an X-ray diffraction chart. This X-ray diffraction chart is shownin FIG. 14.

It is appreciated that there is a Cu peak in the X-ray diffraction chartof FIG. 14. The surface of the diamond was polished to have a mirrorplane. Then the substrate was cut to the size of 2 mm×1 mm×0.6 mm(lengthwise×breadthwise×thickness). A portion of the thin diamond filmlayer peeled off the substrate.

EXAMPLE 16

A porous body 310 as shown in FIG. 18 in the size of 100 mm×80 mm×1 mm(lengthwise×breadthwise×thickness), formed of a tungsten metal sinteredcompact with the porosity of 21% by volume was prepared. The surface ofporous-body 310 including a hole 311 was subjected to a scratchingprocess with diamond grains. Then, porous body 310 was mounted onsubstrate holder 27. Then, current was conducted from AC power supply 24to tungsten filament 25, whereby the temperature of tungsten filament 25was set to approximately 2050° C.

Then, mixture gas of hydrogen and methane with the methane concentrationof 1 mole % was introduced into reactor 21 through gas inlet 22. A thindiamond film layer 320 as shown in FIG. 19 was grown on porous body 310over 40 hours under the condition that the pressure in reactor 21 wasmaintained at 70 Torr. The thickness of thin diamond film layer 320 was24 μm. Then, a Cu plate was mounted on a heater for heating. Porous body310 on which thin diamond film layer 320 is formed was mounted on thisCu plate.

The Cu was heated using the heater up to the temperature of 1100° C. tobe melted, whereby the Cu permeated into hole 311 of porous body 310over the period of ten hours to form a substrate. The substrate includesthe porous body and Cu. Then, the substrate was cut to the size of 10mm×10 mm×1 mm (lengthwise×breadthwise×thickness). The thin diamond filmlayer was polished to have a mirror plane. The substrate was cut to thesize of 2 mm×1 mm×1 mm lengthwise×breadthwise×thickness) to obtain aheatsink. The thin diamond film layer did not peel off the porous body.The thin diamond film layer and the substrate were subjected tometallization to obtain a heatsink shown in FIG. 20.

Referring to FIG. 20, a heatsink 300 is formed of porous body 310 andthin diamond film layer 320. Porous body 310 is formed of a tungstenmetal sintered compact which is a sintered version of fine tungstenmetal powder. A plurality of holes 311 are present in porous body 310.All these holes 311 communicate with each other. Holes 311 are filledwith Cu 312. At the surface layer 310 a of the porous body, diamond filmlayer 320 permeates into hole 311.

A polycrystalline diamond film layer 320 is formed at the surface ofporous body 310.

A laser diode was installed on this heatsink 300. This laser diodeexhibited oscillation. It is therefore appreciated that this heatsink issufficient for actual practice.

EXAMPLE 17

A porous body of 100 mm×80 mm×2 mm (lengthwise×breadthwise×thickness)insize, formed of a porous tungsten metal sintered compact with theporosity of 28% by volume was prepared. The surface of porous body wassubjected to a scratching process with diamond grains. Then, the porousbody was mounted on substrate holder 27 shown in FIG. 1. The temperatureof tungsten filament 25 was set to approximately 2100° C. Mixture gas ofhydrogen and methane with the methane concentration of 1 mole % wasintroduced into reactor 21 through gas inlet 22. The pressure in reactor21 was set to 70 Torr. A thin diamond film layer was formed on theporous body over 40 hours under the above conditions. The thickness ofthe thin diamond film layer was 22 μm.

Then, the porous body was placed on substrate holder 27 with the thindiamond film layer downwards. A Cu plate was placed on the porous body.A heater was placed on this Cu plate to heat the Cu plate up to thetemperature of 1100° C. to melt the Cu. Accordingly, the Cu permeatedinto the porous body to form a substrate. The substrate includes theporous body and Cu. Then, the substrate was cut to the size of 10 mm×10mm×2 mm (lengthwise×breadthwise×thickness). The thin diamond film layerhad its surface polished to have a mirror plane. Then, the substrate wascut to the size of 2 mm×1 mm×1 mm (lengthwise×breadthwise×thickness).However, the thin diamond film layer did not peel off the substrate.

The plane of the substrate opposite to the plane where the thin diamondfilm layer is formed was polished and subjected to metallization toproduce a heatsink. A laser diode was installed on this heatsink. Thislaser diode exhibited oscillation. It is therefore appreciated that thisheatsink is applicable to practice usage.

EXAMPLE 18

A porous body of 100 mm×80 mm×2 mm in size(lengthwise×breadthwise×thickness) formed of a porous tungsten metalsintered compact with the porosity of 35% by volume was prepared. Thesurface of the porous body was subjected to a scratching process withdiamond grains. This porous body was placed on substrate holder 27 shownin FIG. 1. The temperature of tungsten filament 25 was set toapproximately 2100° C. Mixture gas of hydrogen and methane with themethane concentration of 1 mole % was introduced into reactor 21 throughgas inlet 22. The pressure of reactor 21 was set to 70 Torr. A thindiamond film layer was grown on porous body over 40 hours under theabove conditions. The thickness of the thin diamond film layer was 22μm.

Then, a crucible was prepared as a container. Cu was introduced intothis crucible. The Cu was heated up to the temperature of 1100° C. to bemelted. The porous body was dipped in the crucible to have the Cupermeate into the hole of the porous body to form a substrate. Thesubstrate includes the porous body and Cu. Then, the substrate was cutto the size of 10 mm×10 mm×2 mm lengthwise×breadthwise×thickness). Thesurface of the thin diamond film layer was polished to have a mirrorplane. Then, the substrate was further cut to the size of 2 mm×1 mm×1 mm(lengthwise×breadthwise×thickness). The plane of the substrate oppositeto the plane where the thin diamond film layer is formed was polishedand subjected to metallization to produce a heatsink. A laser diode wasinstalled on this heatsink. This laser diode exhibited oscillation. Itis therefore appreciated that this heatsink is applicable to practiceusage.

EXAMPLE 19

A radiator base 81 of 20 mm×20 mm×0.4 mm(lengthwise×breadthwise×thickness) formed of Si, AlN, CuW alloy or SiCas shown in FIG. 21 was prepared. One surface thereof was subjected to ascratching process using diamond powder. Then, diamond was grownentirely on that surface by hot filament CVD. The conditions for growthis set forth in the following.

Material gas 1% by weight methane-hydrogen Flow rate 600 sccm Pressure80 Torr Substrate Temperature 710° C. Filament Tungsten FilamentTemperature 2150° C.

At one surface of each radiator base 81, a vapor synthesis diamond layer82 of high adherence was obtained. Each vapor synthesis diamond layer 82was polished to have respective thickness of 20 μm, 50 μm, and 100 μm.The thermal conductivity of each obtained vapor synthesis diamond layer82 was measured by the laser flash method. Each vapor synthesis diamondlayer 82 had the thermal conductivity of 1310 W/m·K.

Following the polishing process, each vapor synthesis diamond layer 82was cut by laser and had its surface subjected to metallization. All themetallized layers were Au3 μm/Pt0.05 μm/Ti0.1 μm. The opposite surfaceof radiator base 81 was metallized by depositing Au, and bound to a basemetal member 83 formed of a CuW alloy with a brazing material. An MMICformed of GaAs which is a semiconductor element 84 is bound using abrazing material to the metallized layer on vapor synthesis diamondlayer 82 with the heat generating region above.

As a comparative example, a diamond free-standing plate (20 mm×20 mm×0.4mm: lengthwise×breadthwise×thickness) and a BeO substrate (20 mm×20mm×0.4 mm: lengthwise×breadthwise×thickness) were prepared instead ofthe heatsink formed of a layered member of the above vapor synthesisdiamond layer 82 and radiator base 81. In a similar manner, the basemetal member and the semiconductor element (MMIC) were bound.

Sample 13 using the diamond free-standing plate was cracked when thesemiconductor element of GaAs was mounted. In contrast, respectivesamples 1-12 of the present invention exhibited no breakage of thesemiconductor element at the time of mounting by brazing. This isbecause the thermal expansion coefficient of samples 1-12 of the presentinvention is similar to that of GaAs. The mounted semiconductor element,i.e., MMIC, operated stably. Samples 1-12 and 14 that did not exhibitbreakage of the semiconductor element have low thermal resistance, asindicating in the following Table 1. Particularly, the thermalresistance of samples 1-12 of the present invention was lower than thatof sample 14 using BeO.

TABLE 1 Diamond Thermal Thickness Resistance Sample Heatsink Structure(μm) (° C./W) 1 Diamond/Si 20 3.71 2 Diamond/Si 50 3.51 3 Diamond/Si 1003.27 4 Diamond/AIN 20 3.59 5 Diamond/AIN 50 3.38 6 Diamond/AIN 100 3.107 Diamond/CuW 20 3.52 8 Diamond/CuW 50 3.30 9 Diamond/CuW 100 2.99 10 Diamond/SiC 20 3.85 11  Diamond/SiC 50 3.65 12  Diamond/SiC 100 3.34 13*Diamond Free-standing plate 400 — 14* BeO Substrate — 4.35 (Note) Samplewith *in table indicates comparative example

EXAMPLE 20

In the manner similar to that of the above Example 19, a thin diamondfilm layer was grown by vapor synthesis on one surface of a radiatorbase 91 of 14 mm×14 mm×0.4 mm (lengthwise×breadthwise×thickness) in sizeas shown in FIG. 22. This thin diamond film layer was polished. Aheatsink was produced having vapor synthesis diamond layer 92 of 40 μmin thickness on radiator base 91.

A polyimide-Cu multilayer interconnection layer (three layers) 95 wasprovided on vapor synthesis diamond layer 92 of the heatsink. Then,excimer laser was focused on a predetermined position to carry out viahole processing to form an interlayer interconnection by the throughhole. Then, similar to Example 19, the radiator base 91 side of theheatsink was attached by brazing to metal member 93 based on CuW. MMICsemiconductor element 94 was bound by brazing at the element mountingarea of vapor synthesis diamond layer 92. Then, connection betweensemiconductor element 94 and multilayer interconnection layer 95 waseffected.

MMIC semiconductor element 94 mounted on this heatsink did not exhibitbreakage during the stage of mounting. Semiconductor element 94 operatedstably for a long period of time. Superior heat dissipation wasexhibited.

The following issues are identified from the above Examples 19 and 20.In order to prevent the semiconductor element from cracking, thesemiconductor module of the present invention includes a radiator baseof 200-700 μm in thickness with the thermal conductivity of at least 100W/m·K, a vapor synthesis diamond layer of 10-200 μm in thicknessprovided on the radiator base, and a high power semiconductor elementmounted on the vapor synthesis diamond layer.

In the semiconductor module of the present invention, the radiator baseis formed of at least one type selected from the group consisting of Si,SiC, AlN, CuW alloy, CuMo alloy, and CuMoW alloy. The base may have theCu concentration become lower as a function of approaching the surface,as used in Examples 1-18, or may have Cu permeated into the hole of a Wporous body. The vapor synthesis diamond layer has a thermalconductivity that is preferably at least 100 W/m·K.

As a specific structure related to mounting a semiconductor element forthe semiconductor module of the present invention, at least a portion ofthe semiconductor element mounting surface of the vapor synthesisdiamond layer has a metallized layer, wherein the metallized layer isformed of at least one type selected from the group of Au, Mo, Ni, Pt,Pd, Ti, Cu, Al, and has a high power semiconductor element bound on themetallized layer by a brazing material.

In the semiconductor module of the present invention, the mounted highpower semiconductor element has gallium, arsenide as the main component.The semiconductor module of the present invention is particularlysuitable for mounting a high power transistor and a MMIC. The high powersemiconductor element preferably has its plane opposite to the heatgenerating region bound to the vapor synthesis diamond layer side.

In the semiconductor module of the present invention, the packagingdensity of the semiconductor element can be further improved by forminga multilayer interconnection layer including an insulation layer havinga dielectric constant of not more than 5 and a metal interconnectionlayer at the plane side of the vapor synthesis diamond layer where thehigh power semiconductor element is mounted.

In the semiconductor module of the present invention, a thin vaporsynthesis diamond layer of 10-200 μm in thickness is arranged at theplane side where the semiconductor element of the radiator base servingas a heatsink or a radiator base is mounted. By arranging such a thinvapor synthesis diamond layer on the radiator base, the heat generatedfrom the semiconductor element mounted on this vapor synthesis diamondlayer can be dissipated efficiently. Also, the semiconductor element canbe prevented from being cracked at the time of mounting.

The heat generated by the semiconductor element is first diffusedlaterally within the vapor synthesis diamond layer of high thermalconductivity to be further diffused towards the radiator base from theentire surface of the vapor synthesis diamond layer. Therefore, heatdissipation of high efficiency can be applied to the module even if thevapor synthesis diamond layer is thin. The thermal conductivity of thevapor synthesis diamond layer is preferably at least 100 W/m·K in orderto achieve favorable heat dissipation in the horizontal direction in thevapor synthesis diamond layer.

Although the heat conductivity of the radiator base may be smaller thanthe heat conductivity of diamond, the heat dissipation effect of thevapor synthesis diamond layer will be lost if the thermal conductivityis too low. Therefore, the thermal conductivity of the radiator basemust be at least 100 W/m·K. As a radiator base with such thermalconductivity, Si, SiC, AlN, Cu, CuW alloy, CuMo alloy, CuMoW alloy andthe like is preferably used.

In general, diamond is degraded in mechanical strength as it becomesthinner, and has a low thermal expansion coefficient. There was aproblem that the semiconductor element is cracked due to difference inthermal expansion when a semiconductor element formed mainly of galliumarsenide (GaAs) such as a MMIC is connected by brazing.

However, the present invention provides a thin coat of diamond that isless than 200 μm in thickness by vapor synthesis on a radiator base thathas a thermal expansion coefficient higher than that of diamond.Accordingly, the obtained vapor synthesis diamond layer has a thermalexpansion coefficient approaching that of the radiator base withouthaving the mechanical strength degraded. Therefore, cracking of thesemiconductor element can be suppressed even when a semiconductorelement of GaAs is brazed on the vapor synthesis diamond layer.

When the thickness is at least 10 μm, the vapor synthesis diamond layerhas sufficient mechanical strength and exhibits sufficient heatdissipation. Preferably, the thickness of the vapor synthesis diamondlayer is at least 20 μm. However, the cost will increase as the vaporsynthesis diamond layer becomes thicker. If the vapor synthesis diamondlayer becomes thicker than 200 μm, the influence of the underlyingradiator base will be lost, to degrade the thermal expansioncoefficient. As a result, the possibility of breakage in thesemiconductor element when mounted becomes higher. If the thickness ofthe radiator base is less than 200 μm, the mechanical strength willbecome too weak. If the thickness of the radiator base exceeds 700 μm,the entire heat dissipation of the module will be degraded. Therefore,the thickness of the radiator base is preferably in the range of 200-700μm, more preferably in the range of 250-500 μm.

Natural diamond or high pressure synthetic diamond can be used for thediamond layer provided on the radiator base. However, it is difficult tobond the diamond layer and the base together. Degradation in heatdissipation Will occur. There was a problem that a diamond layer of alarge area cannot be produced. According to the vapor synthesis method,a thin diamond film layer can be directly grown on the radiator base.According to the present invention, a vapor synthesis diamond layer withthe required heat conductance and thickness can be obtained easily.Furthermore, the cost can be reduced considerably in comparison to thecase using natural diamond or high pressure synthetic diamond.

The layered member of the vapor synthesis diamond layer and the radiatorbase is used as the heatsink or the radiator substrate. A high powersemiconductor element such as a high power transistor or a MMIC ismounted on the vapor synthesis diamond layer. These high powersemiconductor elements have GaAs as the main component. Thesemiconductor element generally has a heat generation region at onesurface side. The semiconductor element can be mounted with the surfaceopposite to the side of this heat generation region facing the vaporsynthesis diamond layer mounted.

In order to mount a semiconductor element, a metallized layer is formedof at least one type selected from the group consisting of Au, Mo, Ni,Pt, Pd, Ti, Cu, Al, and the like for the semiconductor element mountingplane of the vapor synthesis diamond layer. A high power semiconductorelement such as a high power transistor or a MMIC is fixed on themetallized layer by a brazing material such as AuSn, AuGe, and AuSi. Thetotal thickness of these metallized layer and brazing layer ispreferably in the range of 0.1-50 μm.

The packaging density of the semiconductor element can further beimproved by interconnection with the semiconductor element according toa multilayer interconnection layer of an insulation layer and a metalinterconnection layer formed at the plane of the vapor synthesis diamondlayer where the semiconductor element is mounted. In this case, theinsulation layer preferably has a dielectric constant of not more than 5since an insulation of lower dielectric constant can have the noise bypropagation or the loss reduced.

The layered member of the above vapor synthesis diamond layer andradiator base is connected to the general base metal such as CuW at theradiator base side to be used as a heatsink or a radiator substrate. Inthis case, a metallized layer formed of at least one type selected fromthe group consisting of Au, Mo, Ni, Pt, Pd, Ti, Cu, Al, and the like isprovided at the surface side of the radiator base where the base metalis connected. The metallized layer is bound using a brazing materialsuch as AuSn and AuSi.

According to the present invention, a heatsink can be provided improvedin adherence between the thin diamond film layer and the substrate, withthe warping reduced.

The present invention is advantageous in that the fabrication is carriedout without using toxic BeO. Since a thin vapor synthesis diamond layeris used, the cost can be suppressed. A semiconductor module superior inheat dissipation can be provided. The semiconductor module hasdifference in thermal expansion with the semiconductor element reducedby the radiator base and the vapor synthesis diamond layer formedthereon to alleviate thermal stress in the element. Therefore, crackingin the semiconductor element can be suppressed during the elementmounting stage.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the spiritand scope of the present invention being limited only by the terms ofthe appended claims.

What is claimed is:
 1. A heatsink fabrication method comprising thesteps: a) immersing a surface of a substrate including Cu and a metalhaving a low thermal expansion coefficient in an acid to reduce a Cucontent of a region of a surface layer of said substrate, and to exposeand roughen an exposed surface of said metal at said region, whereinsaid step a) is carried out until a portion of said substrate within 30μm in depth from said surface of said substrate at said region has aporosity of at least 5% by volume and not more than 70% by volume, andhas a Cu content that is not more than 50% of an entire Cu content ofsaid substrate; and b) forming a thin diamond film layer by vaporsynthesis on said region of said surface layer.
 2. The heatsinkfabrication method according to claim 1, wherein said acid is an acidsolution selected from the group consisting of hydrochloric acid, nitricacid, sulfuric acid, hydrofluoric acid, hydrogen peroxide, chromic acid,and a solution of a mixture thereof.
 3. The heatsink fabrication methodaccording to claim 1, wherein said step a) includes a first acidtreatment step of immersing said surface of said substrate in a firstacid, and a subsequent second acid treatment step of immersing saidsurface of said substrate in a second acid different from said firstacid.
 4. The heatsink fabrication method according to claim 1, whereinsaid substrate comprises a sintered compact selected from the groupconsisting of a Cu—W sintered compact and a Cu—W—Mo sintered compact. 5.The heatsink fabrication method according to claim 1, wherein saidsubstrate includes W particles that are exposed at said surface of saidsubstrate that has been immersed in acid, and wherein said W particleshave a surface roughness R_(Z) of at least 0.05 μm.
 6. The heatsinkfabrication method according to claim 1, wherein said step a) to reducesaid Cu content is carried out until a Cu peak is not detected in anX-ray diffraction chart obtained by irradiating said surface of saidsubstrate with an X-ray.
 7. The heatsink fabrication method according toclaim 1, further comprising a step of scratching said surface of saidsubstrate using diamond, prior to said step b).
 8. A heatsinkfabrication method comprising the steps: a) forming a thin diamond filmlayer on a first surface area of a porous body having a low thermalexpansion coefficient, and b) filling a hole in said porous body with Cuafter said step of forming said thin diamond film layer, wherein saidhole opens at a second surface area of said porous body different fromsaid first surface area.
 9. The heatsink fabrication method according toclaim 8, wherein said porous body comprises a sintered compact selectedfrom the group consisting of a W sintered compact and a W—Mo sinteredcompact.
 10. The heatsink fabrication method according to claim 8,wherein said porous body has a porosity of at least 15% by volume andnot more than 60% by volume.
 11. The heatsink fabrication methodaccording to claim 8, wherein said step of filling said hole in saidporous body with Cu includes permeating molten Cu into said hole in saidporous body.
 12. The heatsink fabrication method according to claim 8,wherein said step of filling said hole in said porous body with Cuincludes placing said porous body onto a solid Cu and then heating andmelting said solid Cu to permeate molten Cu into said hole.
 13. Theheatsink fabrication method according to claim 8, wherein said step offilling said hole in said porous body with Cu includes placing a solidCu onto said porous body and then heating and melting said solid Cu topermeate molten Cu into said hole.
 14. The heats ink fabrication methodaccording to claim 8, wherein said step of filling said hole in saidporous body with Cu includes storing molten Cu in a container, andpermeating said molten Cu into said hole by immersing said porous bodyinto said molten Cu.
 15. The heatsink fabrication method according toclaim 8, further comprising scratching said first surface area of saidporous body using diamond, prior to said step of forming said thindiamond film layer.